In recent years, a battery pack using a secondary battery such as a lithium ion battery is mounted to a portable electronic device such as a cellular phone, a digital camera, etc. In a lithium ion battery, it is generally difficult to detect an amount of electric power remaining in the battery (hereinafter, referred to as a battery remaining amount) from a battery voltage. For this reason, in many cases, a battery remaining amount is measured by detecting a charge and discharge current of a battery and integrating the detected charge and discharge current.
In order to measure a battery remaining amount of a lithium ion battery according to the above-mentioned method, a battery monitoring module is provided to a battery pack in many cases. A semiconductor integrated circuit device constituting a battery monitoring module includes analog circuits such as a high-accuracy A/D conversion circuit, etc., and digital circuits such as a CPU and a timer for integrating measured current values.
FIG. 1 is a block diagram of an example of a current measurement apparatus in a battery monitoring module. In FIG. 1, a current analog value detected by a current sensor 10 is converted by a delta-sigma converter 11. The delta-sigma converted current value is digitized by being subject to a decimation filter process in a decimation filter processing part 12, and the digitized current value is stored in a RAM 13. Thereafter, a gain offset correction process is applied to the current value stored in the RAM 13 in the gain offset correction processing part 14.
Here, there is known a technique to reduce a current ripple upon reception of an output instruction current of a rectangular waveform (for example, refer to Patent Document 1). According to this technique, an LC value of an LC filter circuit is kept small by disconnecting a reactor when a current change rate of a pulse current output is large, and the LC value of the LC filter circuit is kept large by connecting the reactor when the current change rate of the pulse current output is small.
Moreover, there is suggested a technique to suppress a fluctuation in a steady state by filtering a given signal by an adaptive filter (for example, refer to Patent Document 2). According to this technique, a fluctuation state of the given signal is judged using a wavelet transform to set inner parameters of the adaptive filter in response to the judged fluctuation state.